Aside from the load of sand particles, the sweet potato is typically host to a foreigner within its walls. A symbiotic pea crab enjoys a commensal relationship with its host cucumber, meaning that the crab benefits while the host reaps zilch. Though not considered parasitic (the crab comes and goes as it pleases), the potato isn’t always unharmed because the pea crab must enter and exit the “safe house” through the host’s narrow rectum, which is found at the end of a slender, siphonlike "tail." A disconcerting visual to be sure. Adding to the dicey maneuver, the opening is not especially elastic, because tiny bones are embedded in the skin. That the bones are fused make for a relatively rigid opening, and as a result, a crab forcing its way in may damage or kill the sea cucumber. Fortunately, the host isn't a flophouse because only one sexually mature crab holes up at any given time. Considering the host's risk without benefit, it’s understandable why the cucumber acts to thwart a crab from entering. The potato either blocks entry by digging its tail end into the sediment or tries to dislodge the crab by sticking out the rear end and shaking it from side to side. Sometimes the sweet potato is successful. Otherwise, the pea crab makes a beeline for the intestine to help itself to a meal painstakingly collected and digested by the host. For the pea crab, the end truly justifies the means.
Another symbiont, a tiny, eulimid snail sequestered in a pointy white shell, is not as benign as the pea crab. The minute-sized tyrant takes up residence on the sweet potato's smooth surface by drilling its noselike organ through the rubbery skin, then siphoning off the host's nutritious body fluids. Once the snail attaches, it is a permanent arrangement.
The life of a sea cucumber that is a sweet potato is spent in sediment familiar to the plantings of its terrestrial alter egos. Though there are differences in environment (under water or topside) and being animal or vegetable, common ground exists. All are periodically plagued by snails and insect pests (insects and crabs are arthropods). Thus, it is fitting (not necessarily for the pea crab) that humans look to for ways to connect our air and water relations because in the end (not only for the pea crab), all of us are citizens of the same planet.
— Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores.